198 CHEMICAL ENGINEERING sledging. This point is subject to investigation in each case, and is one that must be decided upon the merits of the specific conditions obtaining. Power Requirements.—The size of gyratory crushers is usually indicated by an arbitrary number while jaw crushers are designated by inch measurements of the rectangular feed opening. The table given herewith shows the approximate power requirements of various sizes of gyratory breakers. The power figures given should be used with discretion because of the factors which affect power requirements; the principal of these being the hardness of the rock being broken and the method of feeding. On soft rock, minimum power is required and on hard rock the power requirements increase rapidly. It is ordinarily true that breakers are supplied with more power than they require, that is, motors or engines of excessive power are supplied. The various makes of breakers will also have a definite effect upon power requirements so that with those best designed to obviate high friction, a minimum of power would be required. The starting torque of a gyratory breaker is small unless it should be stopped when full of rock. In such a case, it is usually necessary to dig ib out before attempting to start the machine. The same conditions will obtain with jaw breakers. AVERAGE POWER REQUIREMENTS OF GYRATORY BREAKERS Size of opening on each side of spider, inches Maximum cube that will enter breaker, inches Power required, horsepower 7 by 28 7 10 to 20 8 by 34 8 15 to 25 10 by 40 10 20 to 35 12 by 44 12 25 to 40 15 by 55 15 50 to 70 18 by 68 18 65 to 100 21 by 76 21 100 to 140 24 by 99 . 24 125 to 175 36 by 132 36 150 to 200 42 by 162 42 175 to 200 48 by 180 48 200 to 225 54 by 198 54 200 to 250 60 by 216 60 250 to 300 The most favorable operating condition for jaw breakers is when the ratio of size of feed to that of product does not exceed 6:1. When this ratio is exceeded, the efficiency of the machine is greatly reduced, friction is increased and heating almost inevitable; the capacity is reduced to a fraction of normal, and in general the value of the machine is seriously impaired. The amount of material broken will be in proportion to the width of the discharge opening. That is to say that when reducing rock from 18 to 3 in., the machine will handle about three times as much material .as when reducing from 6 to 1 in., all other factors remaining the same. The speed of the breaker directly affects its capacity. The power required depends on the friction that has to be overcome in addition to the actual work of breaking. For the Blake type of breaker, there is one theoretically correct speed based on the law of gravity. The time of stroke of the movable jaw must not exceed the rate of movement of the material due to gravity, but there are mechanical limitations which will reduce the possible speed. An approximation of powe.r required for the operation of Blake breakers is given in the accompanying table.